This invention relates to certain polymer compositions, their preparation and their use as viscosifiers in alkali metal salt solutions which are particularly useful in viscosifying well-drilling fluids for oil field operations at temperatures above 350xc2x0 F.
It is known to use aqueous polysaccharide compositions in well-drilling operations, e.g. in oil and gas wells. Examples of polysaccharides include cellulose derivatives, such as carboxyethylcellulose, carboxymethylcellulose, carboxymethylhydroxyethylcellulose, alkylhydroxyalkylcelluloses, alkylcelluloses, alkylcarboxyalkylcelluloses and hydroxyalkylcelluloses (particularly hydroxyethylcellulose); and microbial polysaccharides such as Succinoglycan polysaccharides, Scleroglucan polysaccharides and Xanthan polysaccharides.
In order to achieve suitable density for use in well-drilling operations, it is conventional for the known aqueous polysaccharide compositions to include water soluble salts, e.g. as described in UK Patent No. 1,549,734. These salts are typically halide salts (e.g. chlorides and bromides) of mono- or divalent cations, such as sodium, potassium, calcium and zinc, e.g. sodium chloride, potassium chloride, calcium bromide or zinc bromide.
One problem faced in drilling operations relates to thermal stability of well service fluids such as completion fluids, work-over fluids and drilling fluids. Temperatures in subsurface formations generally rise approximately 1 degree C. per hundred feet (30 meters) depth. Any aqueous polysaccharide composition has its own characteristic temperature above which it undergoes severe chemical degradation with undesirable reduction of viscosity, thus imposing limitations upon its use in drilling operations below a corresponding depth.
Additives, for example blends of polymeric alkaline materials such as that sold by International Drilling Fluids (UK) Ltd., under the trade mark xe2x80x9cPTS 200,xe2x80x9d have been used to improve thermal stability of aqueous polysaccharide compositions.
Polymer compositions have now been found that have excellent thermal stability and excellent performance characteristics for use in well service fluid applications. Novel polymer/alkali metal carboxylate compositions disclosed here are suitable for use in aqueous well servicing fluids and incorporate high molecular weight copolymer soluble in brine. Adding salt to a solution of polymer in water is a known method of removing the polymer from solution. The highest molecular weights will come out of solution first and, as more and more salt is added, lower and lower molecular weight polymers come out of solution. In contrast, in the novel aqueous polymer compositions disclosed here, the high molecular weight copolymer, further described below, is sufficiently soluble in brine comprising alkali metal carboxylate to develop suitable high temperature viscosity, that is, viscosity at the elevated temperatures encountered by well drilling fluids. Moreover, in accordance with at least certain preferred embodiments, the polymer compositions have xe2x80x9cdurablexe2x80x9d viscosity, that is, advantageous density and excellent high temperature viscosity is maintained even after prolonged working of the well service fluid. The polymer compositions disclosed here comprise water soluble copolymer having functionality which includes at least sulfonate groups and carboxylate groups. The copolymer is sufficiently soluble in saturated and other high concentration brines of alkali and metal salt(s) of carboxylic acid(s), for example, cesium formate brine, to provide polymer compositions having good density and excellent high temperature viscosity for well servicing fluids.
According to one aspect of the present invention, there is provided aqueous polymer compositions comprising the aforesaid copolymer having at least sulfonate group and carboxylate group functionality, dissolved in brine for use as well-service fluids, such as completion fluids, work-over fluids, and drilling fluids. Preferably, such soluble copolymer is polymerization reaction product of acrylamidomethylpropanesulfonic acid or salt thereof with acrylic acid and/or alkali metal salts or ammonium salts of carboxylic acid or other suitable unsaturated carbonyl compounds (e.g. alpha, beta-unsaturated carbonyl compounds).
In accordance with another aspect, an aqueous polymer composition comprises water soluble copolymer having functionality including at least sulfonate groups and carboxylate groups, and alkali metal salt of carboxylic acid.
It will be apparent to those skilled in the art that incidental quantities of other salts, such as sulfates, nitrates and bicarbonates, may also be present in compositions of the invention, and inevitably will be present in compositions where sea water or other naturally occurring brine is used in preparing the composition. If desired the composition may also contain an antioxidant, e.g. 2-mercaptobenzothiazole and/or other suitable additives. When 2-mercaptobenzothiazole is used it may conveniently be incorporated in the same proportion, w/v, as the water soluble copolymer. Those skilled in the art will appreciate that 1% w/v corresponds to a concentration of 10 kgm.sup.xe2x88x923 (10 g/l) of composition. It is advantageous for the pH of the composition to be preferably greater than 8.0, but not higher than 11.5, preferably at least 9.0 and less than 10.5, for reasons of stability and reduced corrosiveness of the composition. The pH may be controlled by addition of suitable reagents, e.g. sodium hydroxide. The presence of a monovalent salt, e.g., a carbonate salt, e.g. potassium carbonate or cesium carbonate, in the composition can enable a suitable buffering effect to be achieved.
The invention also provides a process of preparing an aqueous polymer composition of the invention. In accordance with one aspect, the alkali metal salt of carboxylic acid is added to an aqueous composition, optionally a brine composition, in which the water soluble copolymer is already dissolved and hydrated. In accordance with one preferred embodiment, the alkali metal salt of formic, acetic and/or other suitable carboxylic acid is added to an aqueous composition containing the water soluble copolymer with, when present, other salts and/or other ingredients, etc. In accordance with an alternative embodiment, the polymer composition described above comprising the water soluble copolymer and the alkali metal salt of carboxylate acid, optionally with other dry ingredients, intended for use in a well service fluid, is provided as a dry powder to which water or brine solution is added to hydrate the polymer and dissolve the alkali metal salt of carboxylic acid. In accordance with another alternative embodiment, the water soluble copolymer is added to a brine comprising the alkali metal salt.
In accordance with certain preferred embodiments, the brine employed in an aqueous polymer composition of this invention contains primarily sodium or potassium salt of carboxylic acid, although as mentioned above, other salts may be present in trace or minor amounts. Typically, such brines are substantially saturated solutions of the sodium or potassium salt. In other preferred embodiments, the cesium salt of carboxylic acid is used with only trace or minor amounts of other salts. Such cesium brines provide aqueous polymer compositions of higher density than corresponding compositions employing sodium or potassium brines. More specifically, for example, aqueous polymer compositions of the invention employing cesium brines above about 40% to 43% w/v (i.e., 4 parts by weight of cesium salt to 10 parts by weight of water used in the polymer composition) have density equal to or greater than corresponding compositions employing substantially saturated potassium brine. Thus, in accordance with certain especially preferred embodiments, aqueous polymer compositions of the invention employ the water soluble copolymer in a cesium formate brine or other suitable brine of cesium carboxylate, with at most only trace or minor amounts of other salts (i.e., such as might be contributed by sea water), and the cesium salt is used in an amount from about 40% to fully saturated.
In accordance with another aspect, there is provided the use of a polymer composition of the invention as a well service fluid, especially as a completion fluid, a work-over fluid or drilling fluid in a well-drilling operation. Preferably, the water soluble copolymer has a weight average molecular weight of at least about 1,000,000 preferably from 1,000,000 to 5,000,000, more preferably 1,500,000 to 3,000,000. In that regard, it is a significant advantage of the present invention, that at least preferred embodiments of the aqueous polymer compositions disclosed here provide well servicing fluids which develop high viscosity, i.e., have an improved and retained viscosity at temperatures up to at least 425xc2x0 F. Moreover, it is a notable advantage of the present invention that such preferred embodiments retain their excellent viscosity characteristics at elevated temperature even after extended use as a completion or drilling fluid. In certain preferred embodiments, the copolymer is soluble in a saturated alkali metal carboxylate brine solution, e.g., is a substantially saturated cesium formate solution, in an amount of at least about 4 lbs/bbl, more preferably at least 6 lbs/bbl. Certain such preferred embodiments have retained viscosity at temperatures up to at least about 425xc2x0 F. after 700 hours of extended use as a well service fluid, as tested and defined by xe2x80x9c13 Recommended Practice Standard Procedure for Field Testing Water-Based Drilling Fluidsxe2x80x9d API Recommended Practice 13B-1 (RP 13B-1) First Edition, Jun. 1, 1990 (American National Standard, ANSI/API RP 13B-1-90 Approved: Jul. 12, 1993). Certain such preferred embodiments employing the high molecular weight, water soluble copolymer described above in an amount of 0.05 to 5 wt. % (based on the total weight of all solids components of the polymer composition) and the alkali metal carboxylate in an amount of 95 to 99.95 wt. % (also based on the weight of all solids components) to produce a substantially saturated or other concentration brine, to provide a well-servicing fluid which develops an apparent viscosity greater than or equal to 20 cPs., a plastic viscosity of greater than or equal to 15 cPs, and a yield point of greater or equal to 5 lb./100 ft2 when dissolved in alkali metal formate brine at a concentration of 2 pounds per barrel and measured at 120 degrees F. In accordance with certain highly preferred embodiments, the well-servicing fluid retains at least 50% of its apparent viscosity after roller aging for 30 days at 375 degrees F. and measured at 120 degrees F. Suitable water soluble copolymers can be prepared in accordance with the teachings of published European patent application No. EP 0068189, the entire disclosure of which is hereby incorporated by reference.